1. Fields of the Invention
This invention relates to a ceramic honeycomb structural body to be used for, e.g., a catalyst support for purifying exhaust gases of an internal combustion engine, or a filter for purifying water, etc., and to methods of preparing the same.
2. Description of Related Art
In recent years, with the appearance of stricter emission regulations regarding automobile engines, it has been required to activate a catalyst for purifying exhaust gases early in order to reduce hydrocarbon emission just after starting the engines. To lower the thermal capacity of the ceramic honeycomb structural body having catalyst components is one of means used to provide such early activation of the catalyst. It is required to reduce the thickness of cell walls of said ceramic honeycomb structural body when such means are used. However, the mechanical strength of said structural body decreases with a decrease in the thickness of the cell walls of said structural body.
With respect to means for improvement of the mechanical strength of the honeycomb structural body, it has been proposed, as described in Japanese Publication No. 62-6855, to fill pores in walls, in the neighborhood of an outer peripheral wall among the outer peripheral wall and lattice walls consisting of channel cells, with reinforcing materials of a lower porosity.
However, the honeycomb structural body reinforced by the above-mentioned method has a problem with lower durability. Specifically, when a ceramic honeycomb structural body is located, as a catalyst support for a catalyst for purifying exhaust gases, in the exhaust pipe of engine, said body is exposed repeatedly to thermal changes from a low temperature to a high temperature. Therefore, when it is used, it sometimes suffers delamination between the lattice walls and the reinforcing materials, or a thermal shock fracture due to the thermal history. Furthermore, said delamination and fracture may arise during the cooling process after the firing process for manufacturing said body. It is supposed that such in conveniences result from the difference, in thermal expansion coefficients, between said lattice walls and said reinforcing materials, and the discontinuity between reinforced and non-reinforced areas.
In addition, as mentioned above, when pores are filled with reinforcing materials, the disappearance of pores open to the surface leads to a decrease in the surface area of said body. Therefore, the amount of a supported catalyst or the amount of a supported carrier having large surface area, which is a medium for applying a catalyst to said body, decreases, and then it is difficult to satisfactorily achieve catalytic performance. Further, when pores are filled with reinforcing materials, alkali metals or alkaline earth metals, such as K, Na, Ca etc., are added as a flux component so as to improve the bondability with a matrix. However, these materials cause an increase in the thermal expansion coefficient, and then when thermal changes are repeated, the use of the reinforcing materials is apt to result in heat shock fracture or delamination.
In light of the foregoing circumstances, it is an object of the present invention to provide a ceramic honeycomb structural body, and methods of preparing the same, which can satisfactorily support a catalyst and can demonstrate superior mechanical strength, even if far thinner walls are used.
The above object is achieved according to a first embodiment of the present invention by providing a ceramic honeycomb structural body, comprising: lattice walls formed by a large number of cells which are channels for a fluid; and a peripheral wall which covers the circumference of said lattice walls, an outer peripheral portion of said lattice walls at least located in the vicinity of said peripheral wall containing a denser portion of a lower porosity than that of an inner peripheral portion of said lattice walls which are located inside said outer peripheral portion.
Other and further object, features and advantages of the present invention will appear more fully from the following description.